Processes for the production of polyethylenes using catalysts based on Ti, Mg and halogen are well known in the art. It is also known that such processes are advantageously operated in the presence of alkylaluminium-based activators. Single activators, or combinations of activators may be used. WO 95/17434, for example, describes a catalyst system which uses both a trialkylaluminium (TAA) and dimethylaluminium chloride (DMAC), the DMAC and TAA being present in molar ratios from 30 to 300. WO 01/19879 describes a similar catalyst system, in terms of the components, but at lower DMAC/TAA molar ratio, in particular, in the range between 0.3 and 5.
In general, the use of chlorine-containing activators has a number of potential drawbacks. Perhaps the biggest drawback is the potential to form acids, which can cause significant corrosivity in the presence of moisture, either in the reactor, the flake conveyors, the purge columns or other parts of the downstream process in general, or possibly the resulting polymer.
Some general background can be found in U.S. Pat. No. 4,567,155 which seeks to provide a catalyst system with no active chloride. Further attempts to address the problems of chlorides have included the “passivation” of reactors, such as described in EP 0718322 B1, and the deactivation of halide-containing olefin oligomerisation catalyst systems under conditions which inhibit or limit the decomposition of the catalyst systems, such as described in WO 2004/082827.